People being treated for their HIV infection are less likely to transmit the virus to others.

Most HIV infections come from people who don't know they carry the virus.

At least 200,000 Americans have HIV and don't know it. Many only find out when they develop AIDS.

"Virtually all AIDS cases are preventable, either by preventing infection or by early treatment," Frieden said. "However, 37,000 Americans were diagnosed with AIDS in 2008."

In 2006, the CDC recommended that all Americans get routine HIV tests. Those at high risk of infection should get tested at least once a year, if not more often.

"Forty-four percent of men found to be infected with HIV did not know they were infected, yet half were tested in the past year," Jonathan Mermin, MD, MPH, director of the CDC's HIV/AIDS prevention program, said at the news conference. "This confirms the importance of more frequent testing for those at high risk."

Frieden stressed the importance of making HIV tests routine. He noted that about 70% of people diagnosed with HIV only when they had AIDS had received medical care but were not tested.

"It is not as if these are people there is no hope of reaching," he said. "This is why the CDC recommends routine, voluntary screening for HIV in health care facilities, so even if a person does not indicate an HIV risk factor, we say, 'We test everyone, and we think you should get a test.'"

The CDC report comes in the Nov. 30 "Vital Signs" special issue of Morbidity and Mortality Weekly Report.